The present invention relates in general to mixer circuits and, more particularly, to a series biased single-ended mixer.
Mixer circuits are commonly used in radio applications anywhere it is necessary to perform a frequency conversion between a radio frequency (RF) signal and a local oscillator (LO) signal. The mixing process provides an intermediate frequency (IF) output signal as the sum and difference of the mixing product between the RF and LO input signals. In up-mixer applications, the sum of the mixing products provides the higher frequency output signal. In down-mixing applications, the LO signal is subtracted from the RF signal to produce the IF output signal.
Mixing circuits come in a variety of configurations including single-ended, double-balanced and single-balanced type mixers. The single-ended mixer may include a gallium arsenide field effect transistor that combines the RF signal and LO signal at its gate. A buffer amplifier and filtering circuit may be used to isolate the RF signal from the effects of the LO signal. The RF signal and the LO signal as well as their sum and product terms appear on the drain of the mixing transistor. A filtering capacitor is typically coupled from the drain of the mixing transistor to its source in order to isolate the desired sum and difference terms for the IF output signal.
A common problem with signal-ended mixers found in the prior art is the need to individually supply the buffer amplifier and the mixing transistor with sufficient current for operation. For example, the buffering transistor and mixing transistor may each require 2.0 milliamps of operating current from separate power supply taps. It is desirable for example in battery applications such as cellular telephones and digital pagers to reduce the current flow and power consumption through the mixing circuit.
Another common problem with the single-ended prior art mixer occurs during a disable mode when the mixer should not be operating. The mixing transistor is typically an enhancement mode device with its gate referenced to ground potential. The LO signal may have sufficient power to enable the operation of the mixing transistor. Therefore, the prior art mixer typically uses a switching circuit in the LO signal path to block the LO signal from the gate of the mixing transistor during disable periods to ensure that the device does not function.
Hence, a need exists for a mixing circuit that operates with low power consumption while eliminating the switching circuit in the LO signal path.